Detection of explosives vapor is a challenge, both in terms of the required sensitivity as well as the selectivity. The need for high sensitivity stems from the very low vapor pressure of many of the explosives and from the requirement of detection at some distance from the vapor source. The selectivity issue arises since most of the explosives are not chemically pure and the detection environment contains vapors of many other chemicals.
A number of analytical techniques are being used in commercial trace detectors, such as ion mobility spectrometry (IMS), electron capture mass spectrometry, surface acoustic waves, chemiluminescence and neutron activation. However, current technologies all have serious analytical shortcomings; in particular, the sensitivity for ambient air analysis is not high enough, and therefore it is necessary to swipe luggage at airports as particulates contain much more of the target compounds. The limitations of current technology are well illustrated by the fact that trained dogs can find hidden explosives and drugs, where detectors cannot. A well-known example of this is searching for landmines. Dogs are indeed about 100 times more sensitive than current detectors, and also excel at discriminating one scent from another.
Under development are sensors that are based on adsorption on surfaces. In addition to the plasmon resonance detection, there is an attempt to develop sensors that are based on quartz micro balance (QMB), which changes its frequency upon adsorption of specific molecules, and electrochemical-based sensors.
The most commercially successful detection method is IMS; however, IMS-based sensors require relatively long sampling time and have a relatively high frequency of false positives.
The state of the art laboratory-based technique for trace analysis is mass spectrometry (MS). In combination with gas chromatography and pre-concentration of analyte during sampling, MS can be as much as 100 times more sensitive than dogs. In MS, analyte molecules are ionized and the ions are separated in a mass analyzer according to their masses. A number of different mass analyzers are being used. The most commonly used ionization method is electron ionization (EI) in which molecules in the gas-phase are bombarded with high-energy electrons. MS owes its remarkable sensitivity to the fact that the efficiency of making ions is high, about 0.01%, and that single ions can be detected. MS owes its ability to reliably identify compounds largely to the highly efficient separation of ions in the mass analyzer. However, GCMS traditionally requires very bulky and expensive instrumentation, as well as long analysis times (for gas chromatography). In the last decade, there has been much progress in making mass spectrometers compact and lightweight enough to be field-deployable, and the most attractive design uses time-of-flight (TOF) mass analyzers. These have the simplest possible construction. They are inherently very sensitive, since they allow for all ions to be detected, and have high resolving power. Unfortunately, TOF analyzers are also very difficult to use with present ionization methods, such as EI, for gaseous compounds. As determined for trinitrotoluene (TNT), the state of the art for MS-TOF detection is less than 105 molecules/cm3.
Peroxide-based explosives, in particular, triacetone triperoxide (TATP) and diacetone diperoxide (DADP), are high-powered explosives that can be easily made using inexpensive, readily available starting materials, which can be purchased in most hardware and paints stores, even in bulk quantities. One class of such peroxide-based explosives can be easily produced by reacting various carbonyl compounds, e.g., ketones, aldehydes and their derivatives, with hydrogen peroxide (H2O2) under acid catalysis. For example, when a mixture of acetone, H2O2 and small amounts of a mineral acid such as sulfuric acid is left for several hours at room temperature, white crystals of TATP and DADP are formed. Another commonly used peroxide-based explosive is hexamethylene triperoxide diamine (HMTD), which can be conveniently prepared, e.g., by reacting an aqueous solution of H2O2 and hexamine in the presence of citric acid or dilute sulfuric acid as a catalyst. HMTD is almost insoluble in water and in common organic solvents at room temperature, and it is too active and unstable to be of commercial use as an explosive.
The vapor pressure of peroxide-based explosives, particularly of TATP, is significantly higher than that of commercial explosives, e.g., 8×10−2 torr at 25° C. compared with 6×10−6 ton, for TATP and TNT, respectively. However, the detection of peroxide-based explosives is particularly difficult because unlike most of the commercial explosives, all these materials lack nitro groups or any other nitrogen oxide functional groups. Since most of the currently available explosive detectors are based on the detection of nitro groups, they cannot be employed for detection of peroxide-based materials. Unlike many other explosives, TATP cannot be detected by canines. Furthermore, the current IMS-based detection methods are unable to detect TATP since solid particles of TATP evaporate on a very short time.
Various types of detectors for TATP have already been described (Traversa et al., 2001; Moore, 2004; Pacheco-Londono et al., 2006; Lu et al., 2006; U.S. Pat. No. 7,129,482, Bohrer et al., 2008). In some of these detectors, a trace of the material is detected as disclosed in U.S. Pat. No. 6,767,717 or the detection is alternatively performed by complexation reactions during desorption electrospray ionization (DESI) (Cotte-Rodriguez et al., 2006). A theoretical approach has also been suggested, in which TATP forms a complex with adsorbed molecules (Dubnikova et al., 2002). However, it still remains a challenge to devise a detection scheme that provides the ability to detect TATP vapors with a small size detector, at long distances and with a high sensitivity and selectivity, especially towards H2O2.
International Patent Publication No. WO 98/19151 (corresponding to U.S. Pat. No. 6,433,356) of the same applicant of the present invention, herewith incorporated by reference in its entirety as if fully disclosed herein, describes a hybrid organic-inorganic semiconductor device and sensors based thereon, said device characterized by being composed of: (i) at least one layer of a conducting semiconductor; (ii) at least one insulating layer; (iii) a multifunctional organic sensing molecule directly chemisorbed on one of its surfaces, said multifunctional organic sensing molecule having at least one functional group that binds to the said surface of the electronic device, and at least one other functional group that serves as a sensor; and (iv) two conducting pads on the top layer making electrical contact with the electrically conducting layer, such that electrical current can flow between them at a finite distance from the surface of the device. The semiconductor devices disclosed in WO 98/19151 are referred as molecular controlled semiconductor resistors (MOCSERs) and described as light or chemical sensors.